CHARLOTTE, North Carolina (CNN) --Federal investigators said they know "some sort of catastrophic event" prompted the pilot of a commuter plane to declare an emergency Wednesday moments before the aircraft crashed and burned, killing all 21 aboard.

Investigators probing the crash of Air Midwest Flight 5481 also said they hope the flight recorders from the plane may reveal with what type emergency the pilot was dealing before the aircraft crashed during takeoff.

"We know there was some sort of catastrophic event that caused her (the pilot) to declare an emergency," said John Goglia, head of the 12-member National Transportation Safety Board (NTSB) team heading the investigation.

The flight, operating as US Airways Express, burst into a ball of fire as it crashed shortly before 9 a.m. EST. Fire crews quickly extinguished the flames, and when the smoke cleared a pile of debris could be seen stretching about 100 feet from the site of impact at the Charlotte-Douglas International Airport. (Final path of plane)

The plane, carrying 19 passengers and two crew members, "just barely hit one corner" of the US Airways hangar, setting it on fire, said airport operations director Jerry Orr.

NTSB officials, meanwhile, sent the flight data recorder and the voice recorder to Washington, where investigators would be "working through the night" to get readouts on the recorders, Goglia said.

What investigators learn from the recorders may set the agenda for on-the-site investigations Thursday, Goglia said. "We will focus in on those areas that the boxes give us clues about," he said.

He also said investigators had walked the runway at the Charlotte airport and found aircraft bolts in small pieces. But, he said, "There's no way to determine at the present time if they came from this particular airplane."

As to the possible cause of the crash, Goglia said, "At this point, nothing is out of the question."

A cockpit call to the air traffic control tower before the crash reported some kind of emergency but it was cut off before controllers were able to get details, the Federal Aviation Administration said.

According to reports, among those killed were two graduate students from India who were returning for classes at Clemson University. A freshman from Bob Jones University who was returning to school following the Christmas holiday also died in the crash, the university said.

Medical examiners worked to identify remains Wednesday evening.

Pilot declared emergency

The flight took off at 8:45 a.m. and was due to arrive at Greenville-Spartanburg Airport in South Carolina, 100 miles away, at 9:15 a.m. The day was cold and clear, with a visibility of more than 10 miles, but windy.

The craft could not maintain altitude after taking off and veered to the left, said Orr. Airport officials got a call from the facility's tower at 8:49 a.m., "and we were on the scene within two minutes," he said.

The pilot was Katie Leslie, and the flight's first officer was Jonathan Gibbs, both based in Charlotte, US Airways said. There were no flight attendants on the plane because it was a short flight.

Goglia said it was his understanding that Leslie had 2,500 hours of experience, with 1,800 hours in that particular type of aircraft and Gibbs had 700. He said it is unclear which pilot was flying the plane.

A center had been set up at the Greenville-Spartanburg airport for any friends and relatives of the crash victims who might have been arriving to meet passengers from the flight, a spokeswoman for the South Carolina airport said.

'It went into a nose dive'

Witnesses said plumes of gray smoke could be seen rising from a hangar after the crash.

"I heard a splutter noise," said witness Tommy Stacey. "I saw a plane going straight up into the air, then all of a sudden it went into a nose dive ... and I saw it crash into a ball of fire." (More witness comments)

The FAA identified the aircraft as a Beech 1900D turboprop. It was manufactured in 1996 and had clocked just over 15,000 hours of flight time, Air Midwest said. (Aircraft details)

According to incident reports the FAA released Wednesday, the plane's leaky fuel pump was replaced last fall, and landing gear that would not retract had to be fixed in May. Minor mechanical problems with the aircraft also had been reported, according to the reports.

The agency also said it put out a Beech 1900D maintenance alert in August, telling airlines that a mechanic had found a vertical stabilizer bolt that was loose in one of the aircraft. The alert advised airlines to check the stabilizers on all such aircraft.

The crash comes some 14 months after the last commercial airline fatalities in the United States, federal officials said.

The last incident with fatalities took place on November 21, 2001, when American Airlines Flight 587 crashed in New York, killing 265 people.

It was the fifth fatal crash involving the Beech 1900D since the model debuted in 1991, according to the Aircraft Crash Record Office in Geneva, Switzerland. The most recent happened in August 1999 in Quebec, Canada, killing seven.

The others were in January of that year in Hyannis, Massachusetts, in which nobody died, July 1998 in Vannes, France, in which 14 were killed. A crash in Port au Prince, Haiti, on December 2, 1995, claimed 20 lives, ACRO said.

The airline's hub is in Kansas City, Missouri. Headquartered in Wichita, Kansas, Air Midwest also has hubs in Kansas City; Tampa, Florida; Pittsburgh and Philadelphia, Pennsylvania.